Part 42

Sometimes stars have all at once appeared, shone with a bright light, and vanished. Several instances of these temporary stars are on record. A remarkable one occurred in the year 125, which is said to have induced Hipparchus to form the first catalogue of stars. Another star appeared suddenly near α Aquilæ in the year 389, which vanished after remaining for three weeks as bright as Venus. On the 10th of October, 1604, a brilliant star burst forth in the constellation of Serpentarius, which continued visible for a year; and on the 11th of November, 1572, a star all at once shone forth in Cassiopeia, which rapidly increased in brightness till it surpassed that of Jupiter so much as to be visible at midday. It began to decrease in December of the same year, and, in March, 1574, it had entirely disappeared, having exhibited a variety of tints. It is suspected, however, that this star is periodically variable and identical with stars which appeared in the years 945 and 1264. A more recent case occurred in the year 1670, when a new star was discovered in the head of the Swan, which, after becoming invisible, reappeared, and, having undergone many variations in light, vanished after two years, and has never since been seen. On the 28th of April, 1848, Mr. Hind discovered a star of the 5th magnitude in the constellation Ophiuchus, which was very conspicuous to the naked eye, and where he was certain no star even so bright as the 9th magnitude had ever existed, nor was there any record of such a star. From the time of its discovery it continued to diminish till it became extinct. Its colour was ruddy, and was thought to undergo remarkable changes, probably an effect of its low position, as its polar distance was 102° 39ʹ 14ʺ.

Sir John Herschel discovered very singular variations in the star η of the constellation Argo. It is surrounded by a wonderful nebula, and between the years 1677 and 1826 it varied twice from the 4th to the 2nd magnitude; but in the beginning of 1838 it suddenly increased in lustre, so as to be nearly as bright as α Centauri. Thence it diminished, but not below the first magnitude till April 1843, when it had again increased, so as to surpass Canopus, and nearly equal Sirius in splendour. With regard to this singular phenomenon, Sir John Herschel observes, that “Temporary stars heretofore recorded have all become totally extinct. Variable stars, as far as they have been carefully attended to, have exhibited periodical and regular alternations (in some degree at least) of splendour and comparative obscurity; but here we have a star fitfully variable to an astonishing extent, and whose fluctuations are spread over centuries, apparently in no settled period, and in no regular progression. What origin can we ascribe to these sudden flashes and relapses? What conclusions are we to draw as to the comfort or habitability of a system depending for its supply of light and heat on so variable a source? Its future career will be a subject of high physical interest. To this account I will only add, that in the beginning of 1838 the brightness of this star was so great as materially to interfere with the observations of that part of the nebula surrounding it.” Sir John has also discovered that α Orionis is variable, a circumstance the more remarkable as it is one of the conspicuous stars of our hemisphere, and yet its changes had never been remarked. The inferences Sir John draws from the phenomena of variable stars are too interesting not to be given in his own words. “A periodic change existing to so great an extent in so large and brilliant a star as α Orionis cannot fail to awaken attention to the subject, and to revive the consideration of those speculations respecting the possibility of a change in the lustre of our sun itself, which were first put forth by my father. If there be really a community of nature between the sun and the fixed stars, every proof that we obtain of the extensive prevalence of such periodical changes in those remote bodies adds to the probability of finding something of the kind nearer home. If our sun were ever intrinsically much brighter than at present, the mean temperature of the surface of our globe would of course be proportionally greater. I speak now not of periodical, but secular changes. But the argument is complicated with the consideration of the possible imperfect transparency of space, which may be due to material non-luminous particles, diffused irregularly in patches analogous to nebulæ, but of great extent—to cosmical clouds, in short, of whose existence we have, I think, some indication in the singular and apparently capricious phenomena of temporary stars, and perhaps in the recent extraordinary increase, and hardly less sudden diminution, of η Argûs.” Mr. Hind has come to the same conclusion with Goodricke and Sir John Herschel, that the changes in the variable stars are owing to opaque bodies revolving round them; indeed there are strong reasons to believe that there are solar systems analogous to our own in the remote regions of space. Our sun requires nine times the period of Algol to perform a revolution on its axis, while, on the other hand, the periodic time of an opaque revolving body, sufficiently large to produce a similar temporary obscuration of the sun seen from a fixed star, would be less than fourteen hours.

It is possible that the decrease of light in some of the variable stars may arise from large spots on their surface, like those occasionally seen in the radiant fluid masses on the surface of the sun. One of these spots which was measured by Sir John Herschel on the 20th of March, 1836, with its penumbra, occupied an area of 3780 millions of square miles; and the black central part of a spot that appeared on the 25th of May following would have allowed the globe of the earth to drop through it, leaving a thousand miles clear of contact all around this tremendous abyss.

All the variable stars on record of which the places are distinctly indicated have occurred without exception in, or close upon, the borders of the Milky Way, and that only within the following semicircle, the preceding having offered no example of the kind.

Many stars have actually disappeared from the heavens. 42 Virginis seems to be of the number, having been missed by Sir John Herschel on the 9th of May, 1828, and not again found, though he frequently had occasion to observe that part of the sky. Mr. Cooper, of the Markree Observatory, has given a list of fifty stars that are missing since the publication of his list of stars in 1847. Comparing the present state of the heavens with more ancient catalogues, a much greater number have disappeared.

Thousands of stars that seem to be only brilliant points of light, when carefully examined are found to be in reality systems of two or more suns, many of which are known to revolve about one another. These binary and multiple systems are very remote, requiring powerful telescopes to show the stars separately. They are divided into eight classes, according to the proximity of the two stars. The first class comprises only such as are less than 1ʺ of space apart; those of the second class are more apart than 1ʺ and less than 2ʺ, &c. &c. Sometimes the two stars are of equal magnitude, but more frequently a conspicuous star is accompanied by a smaller companion. In some cases the conspicuous star itself is double, as in ζ Cancri, ξ Scorpio, 11 Monocerotis, and 12 Lyncis, which are triple stars. Each of the two stars of ε Lyræ is a beautiful and close double star; so that which in a common telescope appears merely to be a double star, is found to be quadruple with a very excellent instrument. The multiple system of θ Orionis is one of the most remarkable objects in our hemisphere. To the naked eye and with an ordinary telescope it seems to be a single star, but it really consists of four brilliant stars forming a trapezium, and accompanied by two excessively minute and very close companions, to perceive _both_ of which is the severest test of a telescope.

The first catalogue of double stars in which their places and relative positions are given was accomplished by the talent and industry of Sir William Herschel, who made so many great discoveries, and with whom the idea of their combination in binary and multiple systems originated; and that important fact he established by the discovery of a revolving motion in 50 or 60, and by the determination of the revolution of one star about the other of Castor or α Geminorum, the largest and finest double star in the northern hemisphere. He even assigned the approximate periodic times of this and of several other binary systems. More than 100 stars are now known to be stellar systems. The positions of many hundreds were measured by Sir John Herschel and Sir James South; and the catalogue of the double stars in the northern hemisphere, which have been micrometrically measured, has been increased to more than 6000 by MM. Bessel, Struve, and British astronomers.

Extensive catalogues of double stars in the southern hemisphere have been published by the astronomers in our colonial establishments. To these Sir John Herschel added 1081 during his residence at the Cape of Good Hope: the angles of position and distances of the stars from one another he measured, and found that many of them have very rapid orbital motions. The elliptical elements of the orbits and periodic times of fifteen have been determined by the most eminent astronomers with wonderful accuracy, considering the enormous distances and the extreme delicacy and difficulty of the subject. M. Savary has the merit of having first determined the elements of the orbit of a double star from observation. The difficulty of doing so is great, because the nearest fixed star is 211,000 times farther from the sun than the earth is, and the orbit itself is only visible with the best telescopes; consequently a very small error in observation occasions an enormous error in the determination of quantities at that distance.

In observing the relative position of the stars of a binary system, the distance between them, and also the angle of position, that is, the angle which the meridian, or a parallel to the equator, makes with the line joining the two stars, are measured. The different values of the angle of position show whether the revolving star moves from east to west, or the contrary; whether the motion be uniform or variable, and at what points it is greatest or least. The measures of the distances show whether the two stars approach or recede from one another. From these the form and nature of the orbit are determined. Were observations perfectly accurate, four values of the angle of position, and of the corresponding distances at given epochs, would be sufficient to assign the form and position of the curve described by the revolving star; this, however, scarcely ever happens. The accuracy of each result depends upon taking the mean of a great number of the best observations, and eliminating error by mutual comparison. The distances between the stars are so minute that they cannot be measured with the same accuracy as the angles of position; therefore, in order to determine the orbit of a star independently of the distance, it is necessary to assume, as the most probable hypothesis, that the stars are subject to the law of gravitation, and consequently that one of the two stars revolves in an ellipse about the other, supposed to be at rest, though not necessarily in the focus. A curve is thus constructed graphically by means of the angles of position and the corresponding times of observation. The angular velocities of the stars are obtained by drawing tangents to this curve at stated intervals, whence the apparent distances, or radii vectores of the revolving star, become known for each angle of position, because, by the laws of elliptical motion, they are equal to the square roots of the apparent angular velocities. Now that the angles of position estimated from a given line, and the corresponding distances of the two stars, are known, another curve may be drawn, which will represent on paper the actual orbit of the star projected on the visible surface of the heavens; so that the elliptical elements of the true orbit, and its position in space, may be determined by a combined system of measurements and computation. But, as this orbit has been obtained on the hypothesis that gravitation prevails in these distant regions, which could not be known _à priori_, it must be compared with as many observations as can be obtained, to ascertain how far the computed ellipse agrees with the curve actually described by the star.

γ Virginis consists of two stars of nearly the same magnitude; they were so far apart in the beginning and middle of last century, that they were mentioned by Bradley, and marked in Mayer’s catalogue, as two distinct stars. Since that time they have been continually approaching each other, till in January, 1836, one star was seen to eclipse the other, by Admiral Smyth at his Observatory at Bedford, and by Sir John Herschel at the Cape of Good Hope. A series of observations since the beginning of the present century has enabled Sir John to determine the form and position of the elliptical orbit of the revolving star with extraordinary truth by the preceding method. According to his calculation, it came to its perihelion on the 18th of August of the year 1834. Its previous velocity was so great that the revolving star described an angle of 68° in one year. By the laws of elliptical motion its angular velocity must diminish till it arrives at its aphelion. The accuracy with which the motions of the binary systems are measured, and the skill employed in the deduction of the elliptical elements, are now so great, that the periodic time of γ Virginis, determined by Sir John Herschel and Admiral Smyth from their respective observatories, combined with those of Sir William Herschel, only differ by two years, Sir John having obtained a period of 182 years, Admiral Smyth that of 180. By the aid of more numerous observations Mr. Fletcher has found that the true period is 184·53 years, and that the revolving star passed its perihelion in 1837. It is by such successive steps that astronomy is brought to perfection (N. 232).

Some of the double stars have very long periods, such as ς Coronæ, where the revolving star takes 737 years nearly to accomplish a circuit. Others again have very short periods, as η Coronæ, ζ Cancri, and ξ Ursæ Majoris, whose periodic times are 42·500, 58·91, and 58·26 years respectively: therefore each of these has performed more than one entire revolution since their motions were observed. ζ Herculis, whose periodic time is only about 30-1/4 years, has accomplished two complete circuits, the lesser star having been eclipsed by the greater each time. The first of these two truly wonderful events, of one sun eclipsing another sun, was seen by Sir William Herschel in 1782.

The orbits and periodic times of so many of these binary systems having been determined proves beyond a doubt that sun revolves about sun in the starry firmament by the same law of gravitation that makes the earth and planets revolve about the sun (N. 232).

Since the parallax of 61 Cygni and that of α Centauri have been determined, Sir John Herschel has made the following approximation to the dimensions of their orbits and masses. The distance between the two stars of 61 Cygni, that is the radius vector of the revolving star, has hardly varied from 15ʺ·5 ever since the earliest observations; while in that time the star has moved through 50°; it is evident therefore that the orbit must be nearly circular. It is at right angles to the visual ray, and the periodic time is 514 years. The parallax or radius of the earth’s orbit as seen from the star is 0ʺ·348, while the radius of the star’s orbit as seen from the earth is 15ʺ·5; hence the radius of the star’s orbit is to that of the earth’s orbit as 15ʺ·5 to 0ʺ·348, or nearly as 45 to 1. So the orbit described by the two stars of 61 Cygni about one another greatly exceeds that which Neptune describes about the sun. Since the mean distance of the stars and their periodic time are given, the sum of the masses of the two stars is computed to be 0·3529, that of the sun being 1. Thus our sun is not vastly greater nor vastly less than the stars composing 61 Cygni, which is a small inconspicuous star to the naked eye, not exceeding the 6th magnitude.

Of all the double stars α Centauri is the most beautiful: it is the brightest star in the southern hemisphere, equal, if not superior, to Arcturus in lustre. The distance between the two stars has been decreasing at the rate of half a second annually since the year 1822, while the angular motion has undergone very little change, which shows that the plane of the orbit passes through the earth like the orbits of 44 Boötes, and π Serpentarii; that is to say, the edge of the orbit in these three stellar systems is presented to the earth, so that the revolving star seems to move in a straight line, and to oscillate on each side of its primary. Were this libration owing to parallax, it would be annual from the revolution of the earth about the sun; but as years elapse before it amounts to a sensible quantity, it can only arise from a real orbital motion seen obliquely. In this case five observations are sufficient for the determination of the orbit, provided they be exact; but the quantities to be measured are so minute, that it is only by a very long series of observations that accuracy can be attained. In 1834 Captain Jacob determined the periodic time of the revolving star of α Centauri to be 77 years, and the distance between the two to be 17ʺ·5; and since the decrease is half a second annually, the distance or radius vector of the revolving star was 12ʺ·5 in the year 1822; and as Mr. Henderson had determined the parallax or radius of the earth’s orbit as seen from the star to be ·913, it follows that the real semi-axis of the revolving star’s orbit is 13-1/2 times greater than the semi-axis of the earth’s orbit as a minimum. The real dimensions of the ellipse therefore cannot be so small as the orbit of Saturn, and may possibly exceed that of Uranus. It is very probable that an occultation of one of the suns by the other will take place in 1867, or a very close appulse of the two stars.

Singular anomalies have appeared in the motions of 70 Ophiuchi, which was discovered to be a binary system by Sir William Herschel in 1779, and which has since nearly accomplished a revolution. Various orbits have been computed: those which best represent the angles of position fail with regard to the distances of the stars from one another, and _vice versâ_. But it is a very remarkable fact that the errors are periodical, being for considerable periods of time alternately in excess and defect. Captain W. S. Jacob, who determined the periodic time of the revolving star to be 93 years, attributes this anomaly to the disturbing action of an opaque body revolving round the lesser star. Assuming that to be the case, and computing, he found that the errors were considerably diminished both in the angle of position and distance. It is a subject of the highest interest, and well worthy of the attention of such astronomers as have the means of making the necessary observations. Among the triple systems, as ζ Cancri, two of the stars revolve about one another in 58·9 years; but the motion of the third and most distant is so slow, that it has only accomplished a tenth part of its revolution about the other two since the system was discovered.

It appears from the calculations of Mr. Dunlop that ς Eridani accomplishes a revolution in little more than 30 years. The motion of Mercury is more rapid than that of any of the planets, being at the rate of 107,000 miles an hour. The perihelion velocity of the comet of 1680 was 880,000 miles an hour; but, if the two stars of ς Eridani, or of ξ Ursæ Majoris, be as remote from one another as the nearest fixed star is from the sun, the velocity of the revolving star must exceed the power of imagination to conceive. The elliptical motion of the double stars shows that gravitation is not confined to the planetary motions, but that systems of suns in the far distant regions of the universe are also obedient to its laws. The stellar systems present a kind of sidereal chronometer, by which the chronology of the heavens will be marked out to future ages by epochs of their own, liable to no fluctuations from such disturbances as take place in our system. Some stars are apparently double, though altogether unconnected, one being far behind the other in space, as α Lyræ, which apparently consists of two stars, one of the first, the other of the eleventh magnitude. Aldebaran, α Aquilæ, and Pollux are remarkable instances of these optically double stars. It has been shown how favourable that circumstance is for ascertaining the parallax of the nearest of the two. (N. 232.)

The double stars are of various hues: sometimes both stars are of the same colour, as in α Centauri and 61 Cygni, where the larger stars are of a bright orange and the smaller ones a deeper tint of the same, but they most frequently exhibit the contrasted colours. The large star is generally yellow, orange, or red; and the small star blue, purple, or green. Sometimes a white star is combined with a blue or a purple, and more rarely a red and white are united. In many cases these appearances are due to the influence of contrast on our judgment of colours. For example, in observing a double star, where the large one is a full ruby red, or almost blood colour, and the small one a fine green, the latter loses its colour when the former is hid by the cross wires of the telescope. That is the case with γ Andromedæ, which is a triple star, the small one, which appears green, being closely double. ι Cancri is an instance of a large yellow star and a small one which appears blue by contrast. Still there are a vast number where the colours are decidedly different, and suggest the curious idea of two suns, a red and a green, or a yellow and a blue, so that a planet circulating round one of them may have the variety of a red day and a green day, a yellow day and a blue day. Sir John Herschel observes, in one of his papers in the Philosophical Transactions, as a very remarkable fact, that, although red stars are common enough, no example of a solitary blue, green, or purple star has yet been produced.

Sirius is the only star on record whose colour has changed. In the time of Ptolemy it was red; now it is one of the whitest stars in the heavens.

M. Struve has found that, out of 596 bright double stars, 375 pairs have the same intensity of light and colour; 101 pairs have different intensity, but the same colour; and 120 pairs have the colours of the two stars decidedly different.